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Hormones and Heredity by J. T. Cunningham

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northern forms all show a greater development of spines on the scales.
Whether this is an effect of colder temperature it is difficult to say. It
is possible that the difference is due to external conditions, of which
lower temperature of the water is the most obvious, and it may be that
these conditions have a greater effect on the male than on the female in
the Plaice.

Sexual differences in scales, which have a function in the relations of
the sexes, occur in a few other fishes, and these can be attributed with
good reason to mechanical stimulation. For example, in the Rajidae among
Elasmobranchs the males possess on each 'wing' or pectoral two series of
large, recurved, hooked spines. It has been stated, [Footnote: Darwin,
_Descent of Man_ (2nd edit., 1885), p. 331.] apparently by Yarrell, that
these spines are developed only in the breeding season. It is doubtful if
there is any marked breeding season in these fishes, but it is probable
that the spines are absent in the immature male, as it is known that in
_Raia clavata_ the adult male has sharp pointed teeth, while the young
male and the female at all ages have broad flat teeth. It is supposed that
the spines and perhaps the sharp teeth are used for holding the female,
but it seems equally probable that these structures are really used by the
males in fighting with each other. The habits of these marine fish have
not been much observed, but there is little reason to doubt that these
differences in scales and teeth correspond with differences of mechanical
stimulation. This does not at all imply that the scales and teeth
themselves have been produced by mechanical stimulation, or that the
difference between the dermal denticles of Elasmobranchs and the scales of
Teleosteans correspond to differences of stimulation. But the degree of
development of a structure whose presence is due to gametic factors may
very probably be modified by external stimulation, and the modification
may become hereditary. If the views here advocated are true, the two
processes mutation and modification must be always acting together and
affecting the development not only of the individual but of any organ or
structure. Thus the peculiarities of antlers in stags, it seems to me,
prove that the mechanical stimulation due to fighting was the cause of the
evolution of antlers, that without the habit of fighting in the males
antlers would not exist. At the same time each species of the _Cervidae_
has its special characters in the antlers, in shape and branching, and it
would be impossible to attribute these to differences in mode of fighting:
they are due to mutation.

In connexion with the metamorphosis of Amphibia the case of the Axolotl
has always been of very great interest. In the few small lakes near the
city of Mexico where it occurs it has never been known to undergo
metamorphosis but is aquatic throughout its life and breeds in that
condition. Yet in captivity by reducing the quantity of water in which it
is placed the young Axolotl can be forced to breathe air, and then it
undergoes complete metamorphosis to the abranchiate condition. The same
species in other parts of North America normally goes through the
metamorphosis, like other species of the Urodela. It is evident,
therefore, that the Mexican Axolotls, although they have been
perennibranchiate for a great number of generations, have not lost the
hereditary tendency to the metamorphosis which changes the larvae of
_Amblystoma_ elsewhere into an air-breathing terrestrial animal. This may
be regarded as evidence that the conditions of life which prevent the
metamorphosis in the Mexican Axolotl have produced no hereditary effect.
The fact, however, that Axolotls require special treatment to induce
metamorphosis seems to show that they have distinctly less congenital
tendency to metamorphosis than larvae of the same species, _Amblystoma
tigrinum_, in other parts of North America, and this difference must be
attributed to the inherited effect of the conditions. The most important
of these conditions seems to be abundance of oxygen in solution in the
water, and the next in importance abundance of food in the water. Recently
it has been shown that the metamorphosis may be induced by feeding
Axolotls on thyroid gland. But there is no reason to suppose that a
congenital defect of thyroid arising as a mutation was the original cause
of the neoteny, _i.e._ the peisistence of the larval or aquatic,
branchiate condition. Such a supposition would imply that the association
between Axolotls and the peculiar Mexican lakes, supplied with oxygenated
water by springs at the bottom, was purely accidental. Moreover, there is
no evidence that there is any deficiency of thyroid in the Axolotl. The
secretion of the thyroid gland is necessary for the normal growth and
development of all Vertebrates, and we are only beginning to understand
the effects of defect or excess of this secretion. There is nothing very
surprising in the fact that excess in the case of the Axolotl causes the
occurrence of the metamorphosis which had already in numerous experiments
been produced by forcing the animals to breathe air.

Metamorphosis, as in the development of gill arches and gill slits in the
embryos of Birds, Reptiles, and Mammals, exhibits a recapitulation of the
stages of evolution of certain organs. But in the case of other organs the
absence of recapitulation is remarkable by contrast. If, as I believe, the
development of lungs and disappearance of gills was directly due to the
necessity of breathing air, it is difficult to avoid the conclusion that
the terrestrial legs were originally evolved from some type of fishes'
fins by the use of the fins for terrestrial locomotion. Yet neither the
amphibian larva nor the embryo of higher Vertebrates develops anything
closely similar to a fin. There is no gradual change of a fin-like limb
into a leg, but the leg develops directly from a simple bud of tissue. The
larva of the Urodela is probably more primitive than the tadpole of the
Frogs and Toads, and in the former the legs develop while the external
gills are still large, long before the animal leaves the water.

It is possible that the limbs were transformed to the terrestrial type
before the animal itself became terrestrial, the habit of swimming having
been partly abandoned for that of crawling or walking at the bottom of the
water, and the tail being used merely for swimming to the surface to
obtain air. But the condition of the Dipnoi, which possess lungs but do
not walk on land, does not support this supposition, for they possess fins
which are either filamentous or fin-like, having a central axis with rays
on each side. There can be little doubt that the digits of the terrestrial
limb are homologous with endoskeletal fin-rays, but the evolution of the
axis of the limb is not to be ascertained either from development or
palaeontology. The absence of metamorphosis here may perhaps be due to the
fact that the lateral fins ceased to function in the earlier aquatic
stages, only the caudal fin being used for swimming. If this were the case
the absence of metamorphosis in the legs is itself an adaptation, the
disuse of the paired limbs in the larva having caused the earlier fin-like
stages of these limbs to disappear, while the terrestrial leg was
developed later by heredity, just as the legs have disappeared in the
larvae of many insects, though fully developed in the adult.

Metamorphosis of structure in Amphibia and in Flat-fishes corresponds to
the change of conditions of life in the free-living animal. In the case of
the eyes of the Cave-fishes the conditions in respect of absence of light
are constant throughout life, and we find only an embryonic development of
the eye taking place by heredity. The question arises whether, when there
is no embryonic recapitulation, it must be concluded that apparent
adaptations are due to mutation and not to function or external
conditions. One case of this kind is that of the limbs of Snakes, where,
if we except the vestiges of hind limbs in the Pythons, there is no trace
of limbs either in the embryo or after hatching. There are several similar
cases among Reptiles and Amphibia. The Slow-worm (_Anguis fragilis_) is
limbless, and so are the members of the sub-class Apoda among the
Amphibia. In these also rudiments of limbs are entirely absent in the
embryos or larval stages. Considering the recent evolution of Snakes as
compared with the origin of lungs and loss of gills and gill slits in
terrestrial Vertebrates in general, we have here a remarkable contrast
which shows in the first place the difference resulting when the change in
habits and conditions in the one case takes place from one stage of life
to another, and in the other case the new habits are constant throughout
life from the moment of hatching. It seems to me that in the present state
of our knowledge we cannot form a decisive opinion on the question whether
the absence of limbs in such cases is the result of mutation or of
disuse--that is, absence of functional stimulation.

The power of flight is an excellent example of adaptation. It has been
evolved independently in Pterodactyls, Bats, and Birds. In the two first
groups, and to a slight degree in the third, the expanse of the wing is
formed by an extension of the skin into a thin membrane, supported by the
fore-limbs. It is not necessary to argue in detail that the evolution of
this membrane and of the modifications of bones and muscles by which it is
supported and moved, can be satisfactorily explained on the theory that
modifications due to mechanical and functional stimulation are ultimately
inherited. In birds, however, the surface of the wing is supplied chiefly
by feathers, and consideration of the matter affords no reason for
supposing that the evolution of feathers was due to any external or
functional stimulation. It is often stated that the feathers of birds are
a modification of the epidermic scales of reptiles, but investigation does
not fully confirm this statement. The reptilian scales are retained on the
tarso-metatarsal region of the leg in the majority of birds, and it would
be expected, if the view just quoted were correct, that a transition from
scales to feathers would be visible at the ankle-joint. This, however, is
not the case. In fowls some breeds have scaly shanks and others feathered.
In those with scaly legs I have found cases in winch, in the chicks, there
were two or three very minute feathers, and I have examined these
microscopically by means of sections of the skin. The result was to show
that the minute feathers were not a prolongation of the tips or edges of
the scales, but arose from follicles between the scales. The scale is flat
and is a fold of the epidermis not arising from an invaginated follicle.
The feather, on the other hand, is a tubular structure arising from a
papilla at the base of a deep follicle extending inwards from the surface
of the skin. As the feather grows the papilla grows with it. This papilla
consists of vascular dermal, _i.e._ mesodermic tissue, and if the feather
is pulled out during growth bleeding occurs. The epidermic horny tube
splits posteriorly towards the apex of the feather, and is divided into
rachis and barbs, and thus the dermal tissue within, by this time dead and
dry, is exposed and is shed. Every feather is in fact an open wound, and
is perhaps the only other case, in addition to that of the antlers of
stags, in which vascular mesodermic tissue is normally shed in such
considerable quantities. When the development of the feather is complete,
growth gradually ceases, the proximal part of the feather remains tubular
and does not split, and the vascular tissue within dies, shrivels, and
dries up, forming the pith of the quill When the papilla recommences to
grow the old feather is pushed out, and this process causes the moult. It
would appear, therefore, that the feather must have been evolved, not by a
continuous modification from the scale but by a development of a new kind
between the scales. I have been unable to discover hitherto any evidence
suggesting an external stimulus which could cause this remarkable process
of development in feathers, or indicating that the function of flight
would involve such a stimulus. For the present, therefore, we must
conclude that feathers are not an adaptation, and not due to somatogenic
modification, but must be result of a gametogenic mutation.

Feathers, having been evolved, served in the wings and tail as important
organs of flight. There is reason to believe that, once present, the
growth of feathers was modified greatly by the degree of stimulation
applied to the papillae at roots by the movement and bending strain of the
feathers. The modification of the hones and of the wing, shoulders, and
sternum by the functional stimuli involved in flying are obviously
adaptations, and in my opinion are only to be explained as the hereditary
effects of functional stimulation, like all skeleto-muscular adaptations.
The strains produced in bones by muscular contraction produce hypertrophy
of the part of the bone to which the muscles are attached and thus we can
understand the origin of the carina of the sternum in flying birds, and
its absence in flightless forms. In bats and in pterodactyls also the
sternum is produced into a carina along the median line. The reduction of
the digits of the wing in birds to three, with the bones firmly united
together, would follow from their use in flight and their disuse as
digits, and it would seem, from the fact that the flight-feathers must
have been always on the posterior edge of the wing, and that the ulna is
larger than the radius, that the three digits which have persisted are the
3rd, 4th, and 5th, and not the 1st, 2nd, and 3rd as usually taught. A
comparison of the hind-limbs of birds with those of bats and pterodactyls
suggests strongly that the patagium flyers have arisen from arboreal or
climbing animals, while the birds arose from terrestrial forms which
acquired the bipedal habit, as certain reptiles have. An arboreal animal
would necessarily use all four limbs, as climbing animals actually do. The
wings of birds, on the other hand, would have arisen, from the endeavour
to increase speed by movements of the fore-limbs. The perching birds would
therefore have arisen by later adaptations after the power of flight had
been evolved.

Complete recapitulation does not occur in the development of the digits of
the wing. Only a rudiment of a fourth digit has been found in the
embryonic wing, not, as might be expected, rudiments of five digits of
which two disappear. The metacarpals are free, not united as in the adult,
and there are separate distal carpals, which in the adult are united with
the metacarpals. In other respects the modifications of wings and sternum
are so obviously adaptive that it is difficult to believe that the
reduction of digits was not due to disuse. This is another of those cases
in which the function to which structure is adapted is constant from the
beginning of independent life to the end, and there is some ground for
believing that in course of time in such cases embryonic recapitulation
may be much diminished or disappear. The period of time since birds were
first evolved is in all probability immensely greater than that which has
elapsed since the blind fish, _Amblyoysis_, was modified by cave-life, so
that we can understand why the eye is developed to a certain stage in the
embryo of the blind fish, although it lives in darkness all its life,
while embryonic recapitulation in the wing of the bird is very incomplete.

In another class of adaptations the embryonic or larval stage is adapted
to new conditions, while the adult condition is either less changed or not
changed at all. One of the most obvious examples of this is the allantois
in the Amniota. The embryos of Reptiles, Birds, and Mammals all develop
two embryonic or foetal membranes, the amnion and the allantois. Of the
function or origin of the amnion little is known: to state that it is
protective affords little explanation. It seems possible that it is merely
the mechanical result of the weight of the embryo and the development of
the allantois. The latter is a precocious hypertrophy of the cloacal
bladder found in Amphibia, with the function of embryonic respiration. In
the water the amphibian larva respires by means of gills and gill slits.
In adaptation to terrestrial life it is necessary, if the free aquatic
larval stage is to be eliminated, that the embryo should be able to
breathe air before hatching. Various Amphibia show how this requirement
was met in various ways. In the South American tree-frogs of the genus
_Nototrema_ the eggs are developed in a dorsal pouch of the skin of the
female, and within this pouch the respiration of the embryo is carried on
by a membranous expansion of the second and third external gills on each
side. In the Reptilia the bladder is expanded for the same function, and
absorbs oxygen and gives off carbon dioxide through the pores of the
shell. It is impossible to reconcile the conception of mutation with the
adaptive relation between this allantois and the expulsion of the egg
enclosed in a shell on land. The transition probably came about gradually
from the deposition of the eggs in moist places but not in water. In the
midwife toad (_Alytes obstetricans_) the male carries the eggs about
attached to his legs, respiration is effected by enlarged external gills,
and the larvae are hatched in water. In the ancestral reptiles external
gills may have helped at first, until by the enlargement of the bladder
they were rendered unnecessary. In all such cases the absorption of oxygen
must be regarded as the stimulus which caused the enlargement of the
respiratory membrane. As the allantois could not be absorbed or retracted
again into the abdomen, the umbilicus was evolved--that is to say, the
scar formed by the union of the folded edge between the body wall and
amnion surrounding the stalk of the allantois. It would he difficult for a
mutationist to explain how a mutation should affect the development of the
cloacal bladder to such an enormous degree, just when it was required for
embryonic respiration, and cause the sides of the body to unite ventrally
at the time of hatching, cutting off the allantois and the amnion.

T. H. Morgan [Footnote: _A Critique of the Theory of Evolution_, p.18.]
states that a mutation of gametic origin may affect any stage in the
development of the individual. This may be true when there are already
distinct stages in the life history. The more important question is
whether distinct stages can be caused by mutation. It is true that in
heterozygous individuals characters may develop more fully in the adult
stage than in the young. But when we find different stages evidently
adapted to different modes of life, it is impossible to explain them by
mutations affecting different stages of life. In such cases as the larval
stages of Insects we find the larvae have become adapted to new habits
while the adults have remained unchanged, or have evolved quite
independent adaptations. For example, the adults in the chief orders of
Insects have the typical three pairs of legs, while the maggots or grubs
of the Diptera or Hymenoptera have no legs at all, the caterpillars of
Lepidoptera have evolved pseudo-legs on the abdomen, and the larvae of
Coleoptera have the ordinary legs and no more. This is the reverse of
recapitulation: in the case of legless maggots, and caterpillars with
pro-legs, the adult is more similar to the ancestor than the larva. But
the same principle holds, that where functions and habits are different,
there organs are different. No mutationist has yet produced by breeding
experiments a caterpillar without the three pairs of thoracic legs and yet
developing into a moth that had normal three pairs. Morgan, with all his
mutations of the adult _Drosophila_, says nothing of mutants possessing
legs. The only rational conclusion is that legless larvae have lost the
disuse, since those larvae which are destitute of legs do not go in search
of food but either live in the midst of it or are fed by others, and that
the pro-legs of the caterpillar have been developed by the muscular action
of the insect in clinging to leaves. Here again the hormone theory,
although we cannot pretend to understand the matter completely, helps us
to form a conception of the process of heredity and evolution. The disuse
of legs in the larva affects the determinants, so that they remain
inactive in the presence of the hormones produced in the body generally in
this stage. In the adult stage activity of the legs produces hormones
which influence the same determinants in the gametes to develop legs, but
again in the presence of the different hormones which are present in the
body generally in the adult stage. As the habits of larva and adult became
more specialised and contrasted, the change became less and less gradual,
and the intermediate stage, not being adapted to any transitional mode of
life, became an inactive pupa in which the adult organs develop.

In conclusion I will briefly consider the attempts which have been made to
prove the influence of somatic modifications or characters on the gametes
by direct experiment. The method of Kammerer of inducing changes of habit
or structure by conditions, and then showing that the change is in some
degree inherited, has already been mentioned. One obvious criticism of
this evidence is that it seems to prove too much, for it is difficult to
believe that a change produced in individuals would show so much
hereditary effect in their immediate offspring. Two other methods are
conceivable by which the influence of somatic hormones might be evident.
One of these is to graft ovaries or testes from one animal into another
which possesses a certain somatic character, and then to see if the
offspring produced from these gonads shows any trace of the character of
the foreign soma in which it was nourished. C. C. Guthrie [Footnote:
_Journ. Exper. Zool._ (1908), v.] claimed to have done this in his
experiments on hens. He grafted the ovaries of two Black Leghorn pullets
into two White pullets of the same breed, and vice versa. The black and
the white birds bred true when mated to cocks of their own colour. The
black hen with white ovary mated with black cock produced four black
chicks and two black chicks with white legs, the white hen with black
ovary mated with white cock produced some white chicks, some black and
some white with black spots. This is held to prove that the transplanted
ovaries were functional, because they produced evidence of the character
originally belonging to them. On the other hand, the black hen with white
ovary mated with white cock produced nine white chicks, and eleven chicks
which were white spotted with black, and the white hen with black ovary
mated with black cock produced not black chicks but white chicks spotted
with black. This was held to prove that the somatic characters of the
"foster mothers" were transmitted.

Davenport repeated Guthrie's experiments on different fowls, grafting the
ovary from a cinnamon-coloured hen into a white hen, and mating her with a
cinnamon-coloured cock. The chicks were exactly similar to those obtained
from crossing such a cock with a normal white hen, and Davenport concludes
that the engrafted ovary was not functional but had degenerated. It is
known to be almost if not quite impossible to remove the ovary completely
from a hen, owing to its close attachment over the great post-caval vein.
At the same time it is difficult to see how Guthrie could have obtained
black and spotted chicks from a white hen mated with, a white cock if the
grafted ovary from a black hen had not been functional. One point which
Guthrie does not mention, and of which apparently he was not aware, is
that the white of the White Leghorn is dominant to colour, the
heterozygotes not being pure white but white with spots. Thus when he
mated a black cock with a white hen with grafted ovary and obtained
spotted chicks, this would have been the result if the original white
ovary was functional. None of his results prove conclusively the influence
of the soma of the hen into which ovaries were grafted, but would all be
explained if some eggs were derived from the part of the original ovary
not removed in the operation, and others from the grafted ovary.

The grafting of ovaries in Mammals has often been tried, but very rarely
with success. The introduced ovary usually dies and is absorbed. C. Foa
[Footnote: _Arch. Ital. de Bid._ (1901), Tome xxxv.] states that he made
bilateral grafts of ovaries from newborn rabbits into adult rabbits, and
two months after the operation one of the operated females was fecundated
and produced five normal young. In other cases he placed ovaries from
new-born young in positions far from the normal position, such as the
space between the uterus and bladder, and in one case the female so
treated became pregnant, and when killed had a single embryo in one uterus
and no trace of the original ovaries in the normal position. But Foa was
not investigating the influence of somatic characters on ova in the
grafted ovaries, and does not even mention the characters or breed of the
rabbits he used or of the young which were produced from the grafted
ovaries. Castle [Footnote: W. E, Castle and J. C. Phillips, _On Germinal
Transplantation in Vertebrates_, Pub. Carnegie Institution in Washington
(1911), No. 144.] carried out seventy-four transplantations of ovaries
principally in guinea-pigs. Out of all these only one grafted female
produced young. In this case the ovaries of two different black
guinea-pigs about one month old were grafted into an albino female about
five months old. After recovery the grafted female was kept with an albino
male. She produced six young in three pregnancies, first two, then one,
and lastly died with three foetus in the uteri. All these were black, with
some red hairs among the black. One of the first two young had a white
forefoot. In this case black is dominant, and therefore there is nothing
extraordinary in the offspring from a black grafted ovary being black. The
presence of red hairs and a white foot is no evidence of the influence of
the foster soma, but is due to imperfect dominance. When the same male was
mated with a normal black female the offspring were black with red hairs

All these experiments are open to the following criticism. It has been the
main argument of this volume that there are two distinct kinds of
characters in all organisms--namely, those of somatogenic origin and those
of gametogenic origin. Theory supposes that somatic modifications by means
of hormones affect the determinants in the gametes. But it is obvious that
the black and white of Leghorn fowls and of guinea-pigs are gametogenic
characters, and are strongly established in the gametes of their
respective varieties. It is not even certain that the black or white hair
or feathers are giving off special hormones which would or could influence
the gametes. The hormone theory only postulates such influence from
hormones issuing from tissues modified by external stimuli. It is quite
certain that the black colour in Leghorns or guinea-pigs is not due to any
external stimulus or influence. The experiments therefore are entirely
irrelevant to what has been called the inheritance of acquired characters.
All that they can be said to prove is that an albino soma does not convert
ingrafted ova of black race into ova carrying the albino character.

It is probably impossible to prove experimentally the influence of a
modified soma in one generation. I have endeavoured to find a case which
would not be open to the above criticism--that is, to find a character
which could be considered somatogenic and which was absent in a closely
allied variety. Most of the characters in domesticated varieties are
obviously gametogenic mutations, but the lop-ear in rabbits may be, partly
at least, somatogenic. Since many breeds have upright ears, we cannot say
that disuse of the external ear has produced lop-ears in domesticated
rabbits generally, but in lop-eared breeds the ears are much enlarged; and
though this may be gametogenic, the increased weight may have been the
cause of the loss of the power to erect the ears. I therefore tried
grafting ovaries from straight-eared females into lop-eared individuals.
The operation was perfectly successful in seven specimens--that is to say,
they recovered completely and lived for many months, up to a year or more
afterwards, but none of them became pregnant. When killed no trace of
ovary was in any of them; in every case it had been completely absorbed,
and the uteri and vagina were diminished in size and anaemic. For grafting
I used ovaries from young rabbits of various ages from seven days to six
weeks or more, but all were equally unsuccessful. Satisfactory evidence by
direct experiment of the inheritance of somatogenic modifications due to
external stimuli cannot be said to have been yet produced, and, as I have
shown, such evidence from the nature of the case must be very difficult to
obtain. The indirect evidence, however, which has been considered in this
volume is too strong to be ignored--namely, the case of Japanese
long-tailed fowls, that of colour on the lower sides of Flat-fishes, and
the similarity of the congenital development of the antlers in stags, to
the generally admitted effects of mechanical stimulation and injury on the
skin and superficial bones of Mammals.

The general conclusions which are logically to be drawn from our present
knowledge with regard to the problems of heredity and evolution in animals
are in my opinion as follows:--

1. All attempts to explain adaptation by gametogenic mutations, or changes
in gametic factors or 'genes,' have completely failed, as Bateson himself
has admitted.

2. The facts discovered concerning mutations and Mendelian heredity
harmonize with the nature of the majority of specific and varietal
characters, and with the diagnostic characters of many larger divisions in

3. Some of the most striking cases of adaptation, such as the organs of
respiration and circulation in terrestrial Vertebrates, and the asymmetry
of Flat-fishes, are developed in the individual by a metamorphosis which
is generally regarded as a recapitulation of the ancestral evolution. No
cases of mutation or gametogenic variation hitherto described exhibit a
similar metamorphosis or recapitulation.

4. Secondary sexual characters, usually in the male sex, correspond in
their development with the development of maturity and functional activity
in the gonads, and it has been proved that the latter influence the former
by means of 'hormones' or internal secretions. The evidence concerning sex
and sex-linked characters and the localisation of their factors in the
chromosomes of the gametes has no bearing on the action of hormones.

5. The facts concerning the action of hormones are beyond the scope of
current conceptions of the action of factors or genes localised in the
gametes and particularly in the chromosomes. According to these
conceptions, characters are determined entirely by the genes in the
chromosomes, whereas in certain cases the development of organs or
characters depends on a chemical substance secreted in some distant part
of the body.

6. It was formerly stated that no process was known or could be conceived
by which modifications produced in the soma by external stimuli could
affect the determinants in the gametes in such a way that the
modifications would be inherited. The knowledge now obtained concerning
the nature and action of hormones shows that such a process actually
exists, and in modern theory real substances of the nature of special
chemical compounds take the place of the imaginary gemmules of Darwin's
theory of pangenesis or the 'constitutional units' of Spencer.

7. The theory of the heredity of somatogenic modifications by means of
hormones harmonises with and goes far to explain the facts of
metamorphosis and recapitulation in adaptive characters, and also the
origin of secondary sexual characters, their correlation with the
periodical changes in the gonads and the effects of castration. At the
same time there are some somatic sex-characters, _e.g._ in insects and
birds, which do not appear to be correlated with changes in the gonads,
and which are probably gametogenic, not somatogenic in origin.

8. The theory of the heredity of somatogenic modifications is not in
opposition to the mutation theory. The author's view is that are two kinds
of variation in evolution, one somatogenic and due to external stimuli,
acting either directly on passive tissues or indirectly through function,
and the other gametogenic and due to changes in the chromosomes of the
gametes which are spontaneous and not in any way due to modifications of
the soma. Adaptations are due to somatogenic modifications, non-adaptive
diagnostic characters to gametogenic mutations. It is a mistake to attempt
to explain all the results of evolution by a principle. There are two
kinds of congenital, constitutional or hereditary characters in all
organisms, namely, the adaptive and the non-adaptive, and every distinct
type in classification exhibits a combination of the two. To assert that
all characters are adaptive is as erroneous as to state that all
characters are blastogenic mutations, and therefore in their origin

9. Finally it may be urged, although the question has not been directly
discussed in this volume, that no biologist is justified in the present
state of knowledge in dogmatically teaching the lay public that
gametogenic characters are alone worthy of attention in questions of
eugenics and sociology. Hereditary or constitutional factors are of course
of the highest importance, but there exists very good evidence that
modifications due to external stimulus do not perish with the individual,
but are in some degree handed on to succeeding generations, and that good
qualities and improvement of the race are not exclusively due to mutations
which are entirely independent of external stimuli and functional
activity. It is important to produce good stock, but it is also necessary
to exercise and develop the moral, mental, and physical qualities of that
stock, not merely for the benefit of the individual, but for the benefit
of succeeding generations and to prevent degeneration.


_Abraxas groussularioun_ and _lacticolor_
Adaptations, origin of; evolution of
_Agonus entaphractus_
_Alytes obstetricans_
_Amblyopsis_, eyes of
_Amblystoma tigrinum_
_Anableps tetrophthalmus_
_Anas boscas_, crosses of
_Anas tristis_, crosses
Ancel and Bouin
_Anguis fragilis_
_Antirrhinum_, crossing of
Antlers of stags
Ants, heredity of sex in
Aphidae, heredity of sex in
Axolotl, albino; metamorphosis; influence of thyroid feeding

Barred plumage in fowls
Bees, heredity of sex in
Bernard, Claude
Berthold, A. A.
Biedl and Konigstein
Blindness in cave animals
_Bombyx mori_
Boring, Miss
Born and Fraenkel

_Cambarus_, males of
Castle, experiments in grafting; on sex
Castration; in ducks; of frog; of Lepidoptera
Cats, heredity of colour in
Cave animals, absence of pigment
Cetacea, absence of scrotum
_Chologaster agassixii_
Chromosomes; in mutations
Colour-blindness; heredity of
Colours, origin of, in domesticated breeds
Comb of fowls, uselessness of
Corpora lutea, evolution of; in viviparous lower vertebrates; origin of
_Corystes cassivelaunus_
Courtship, organs of
Criss-cross inheritance
Crossing over
Cyclostomes, absence of corpora lutea in
Cytoplasm, in heredity

_Dafila acuta_ crosses
_Daphnia_, heredity of sex in
_Dasyurus_; corpora lutea; lactation
Determination of sex
Dipnoi, fins
Dog-fishes, oviparous and viviparous
Dominant characters, origin of
Doncaster; on heredity in cats
_Drosophila_, blind mutation, heredity of sex, mutations
Ducks, crosses of
Dutch rabbit

Earthworms, sex in
Eclipse plumage
Elasmobranchs; corpus luteum in
Elephants, testes
Evolution, evidence of

Factors, origin of
Feathers, evolution of
Flat-fishes, mutations of
Flight, evolution of
Foa, on lactation; on grafting
Fowls, castration of; origin of breeds
Fractionation of Mendelian factors
Frog, thumb-pad

_Gallus bankiva_
Gates, Dr. R. Ruggles
Geddes and Thomson
Genital ducts
_Gigas, Oenothera_
Gipsy moth
Goltz and Ewald
Gonads, hormones of
Goodale, H. D.
Grafting, of ovaries or testes
Graves' disease
Guthrie, C. C.

Herdwick sheep, castration in
Heredity; and sex
Hill, J. P.

_Inachus scorpio_
Insects, heredity of sex in
Interstitial cells
Intromittent organs

Japanese long-tailed fowls; artificial treatment of


Lactation, dependence on stimulation, in males; regulation of
_Laevifolia, Oenothera_
Lamarckian theory
Lane-Claypon, Miss; and Starling, on ovaries of rabbit
Larvae of insects
_Lata, Cenothera_
Leghorn, White
Leopold and Ravana
Lepidoptera, castration in
_Limantria dispar_
Loeb, on "blind fish; on blindness in cave animals;
on tadpoles and thyroid
Lop-eared rabbits, grafting experiments
Lotsy, Professor; on crossing
Lutein, of corpora lutes

Male characters in female
Mallard crosses
Mammary glands; origin of rudimentary in male
Marshall; and Jolly
Marsupials, relation of foetus to pouch; scrotum of
Masked crab
Meisenheimer; thumb-pad of frog
_Mendel's Principles of Heredity_
Mendelism; and castration
Metamorphosis; in Flat-fishes; causes of; and hormones;
and diagnostic characters
Midwife toad,
Milk glands,
Mole, eyes of,
Monotremata, origin of milk glands,
Morgan, T. H., on blindness in cave animals, on mutations, on sex:,
on sex-linked heredity, on sexual dimorphism in _Drosophila_,
on variation,
Mutations, in antlers,

Natural selection,
Nuptial plumage,
_Nyssia zonaria_

O'Donoghue, development of milk glands,
_OEnothera_, mutations, _grandiflora_, lata_, _Lamarckiana_,
Onagra, species of,
_Origin of Species_, Darwin's,
_Ornithorhyncus_, corpus luteum
Otariidae, scrotum,
Ovaries, position of,
Ovary, in birds,

Pearson, Karl,
Pheasant, male, gynandramorphism in
Phillips, John C.,
_Philosophie Zoologique_
Phoeidae, testes,
_Physiology of Reproduction_,
Picotee Sweet Pea,
Pigment, absence in cave animals,
Pile fowls,
Pintail duck, crosses,
_Pleuronectes flesus_,
Plymouth Rock fowl,
_Problems of Genetics_,
Prong buck,
_Proteus_, eyes of,
Prototheria, milk glands in,

Rabbits, lactation in,
Recapitulation, absence of, and mutations,
Reptiles, corpora lutea in,
Reversal, in Flat-fishes,
_Rhinoderma darwinii_,
Rodents, testes,
Romanes, GJ
Roentgen rays, effect on testes,
Rose comb, in fowls,
Rotifers, heredity of sex in,
_Rubricalyx, Oenothera_,
_Rubrinervis, Oenothera_,

Salamanders, transplantation of eye,
Schuster, Edgar,
Scrotum, origin, of,
Secondary sexual characters,
_Semilata, Oenothera_,
Sertoli's cells,
Sex, chromosomes; Mendelian theory of,
Sex-Linked heredity,
_Sexual Dimorphism_,
Sexual dimorphism, in Rajidae, in Plaice,
Shattock and Seligmann,
Silky fowl, plumage of,
Sirenia, absence of scrotum,
Smith, Geoffrey,
Snakes, absence of limbs,
Somatic sexual characters,
Species, conception of, origin of, characters of, sterility and hybridism,
Spermatogenesis, in man,
Starling and Lane-Claypon, on lactation,
Steinach, heredity of milk glands,
Sternum, carina of,
Sweet Pea,

Tadpoles, effect of thyroid in
Tandler and Gross
Teleosteans; corpora lutea in; ovarian follicles
Testes, descent of
Thumb-pad of frog
Thyroid-gland feeding
Tortoise-shell colour in cats
Tosa fowls, Japanese
Transplantation of gonads

Urodela, larva

_Vespa vulgaris_; _germanica_
Vries, De

Wasps; heredity of sex in
Weapons, organs used as
Whale, paddle of
White Leghorn, crosses
Wilson, E. B.
Wing, development of
Winiwarter, von
Wood, T. B., on crossing of sheep
Woodland, W.

X chromosome


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